1. Technical Field
The present invention relates to a nanofiber spinning method and device for producing nanofibers made of polymeric substances and forming the produced nanofibers into yarn.
2. Description of the Related Art
Conventionally, electrospinning (also referred to as electric charge induced spinning) is known as a method for producing nanofibers made of polymeric substances and having a diameter in submicron order.
In the conventional electrospinning method, a polymer solution is supplied to a needle nozzle to which a high voltage is applied, so that the polymer solution extruded as filaments through the needle nozzle is electrically charged. As a solvent of the polymer solution which is electrically charged evaporates, a distance between these electric charges decreases and Coulomb force acting thereon increases. When the increased Coulomb force exceeds the surface tension of the filamentous polymer solution, the filamentous polymer solution undergoes a phenomenon in which the filamentous polymer solution is explosively stretched. This phenomenon is referred to as an electrostatic explosion. The electrostatic explosion repeats itself as primary, secondary, and sometimes tertiary explosions and so on, and accordingly, nanofibers made of polymers and having a submicron diameter are obtained.
However, since, in the conventional electrospinning method, only a small amount of nanofibers can be produced from the tip of a single nozzle, high productivity cannot be obtained. Consequently, as a method for producing a large amount of nanofibers, a method utilizing a plurality of nozzles has been proposed (For example, see patent reference 1).
According to patent reference 1, polymer solution stored in a barrel is supplied, by a pump, to a plurality of needle nozzles which are electrically charged, and is ejected through the nozzles, thereby producing a large amount of nanofibers. The large amount of nanofibers thus produced, are collected by a collector which is charged to a polarity opposite to those of the nozzles, and transported while being deposited. In such a manner, a highly porous polymer web in which porosity is extremely high, and which is made by nanofibers depositing in a three-dimensional network structure, are produced. Further, the patent reference 1 discloses that such technique improves nanofiber production from a conventional experimental level to a practical level.
Further, conventionally, nanofibers produced by the electrospinning method are formed into a web. Such web is used in various applications, such as an artificial leather, a filter, a diaper, a sanitary pad, an adhesion-inhibiting agent, a wiping cloth, an artificial vessel, and a bone fixation apparatus. However, it is difficult for thus produced nanofiber web to achieve physical properties of 10 MPa or more, which imposes a limitation in a wider range of applications. Further, when forming thus produced nanofibers into a continuous yarn so as to enhance physical properties, there is a problem in that the web has to be cut into a certain length to form short fibers, and the short fibers has to undergo an additional spinning process for forming spun yarns.
Consequently, there is a proposed technique for continuously forming yarn utilizing a nanofiber web produced by the electrospinning method (for example, see patent reference 2). In the patent reference 2, polymer solution is ejected through electrically charged nozzles which are aligned, toward a collector which is charged to a polarity opposite to those of the nozzles. With this, nanofibers are spun on the still surface of water or organic solvent of the collector, and are deposited forming a web. Thus deposited web is pulled by a rotary roller rotating at a constant linear velocity from the position spaced more than 1 cm from one end viewed in the direction of alignment of the nozzles, thereby forming a continuous yarn. Further, the continuous yarn is pressed, stretched, dried and wound so that the continuous yarn which is superior in physical properties can be obtained. The patent reference 2 also discloses that the continuous yarn can also be twisted.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2002-201559    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2006-507428